In digital radiography, an imaging technique is nowadays frequently used in which a radiation image of an object or a patient is temporarily stored in a photostimulable phosphor screen.
The radiation image can then be read out by scanning the screen with stimulating radiation and by detecting the image-wise modulated light which is emitted by the phosphor screen upon stimulation.
Stimulation can for example be performed in a flying spot scanner by scanning the phosphor screen carrying a radiation image by means of a sweeping ray of laser light. Typically a screen is pixel-wise scanned by deflecting the laser beam so that it performs a line-wise movement in a first direction parallel with one of the sides of the phosphor screen. Thereupon the screen is moved in the sub-scan direction perpendicular to the first direction in order to scan the entire surface of the phosphor screen.
When stimulating a pixel of a phosphor that was exposed to a radiation image with light having a wavelength within the phosphor's stimulating wavelength range, the pixel emits image-wise modulated light of a second wavelength. This emitted light can be detected by means of a light sensor such as a photomultiplier and converted in a corresponding electrical signal.
Frequently used stimulable phosphors include divalent europium activated phosphors (e.g., BaFBr:Eu, BaFBrI:Eu) or cerium activated alkaline earth metal halide phosphors and cerium activated oxyhalide phosphors, as well as e.g. a phosphor having the formula of YLuSiO5:Ce,Zr.
Examples of suitable screens are divalent europium activated alkali halide type phosphor screens, wherein said halide is at least one of chloride, bromide and iodide or a combination thereof or divalent europium activated alkaline earth metal phosphor screens wherein said halide is at least one of fluoride, chloride, bromide and iodide or a combination thereof. Specific examples are a divalent europium activated CsX type phosphor screen, wherein said X represents Br or a combination of Br with at least one of Cl and I, as Br(Cl), Br(I) or Br(Cl,I) and a bariumfluorohalide phosphor screen wherein the phosphor is of the (Ba,MII)FX′:Eu type, wherein MII is an alkaline earth metal and wherein X′ is Cl, Br and/or I.
Crystalline divalent europium activated alkali halide phosphor screens advantageously have CsBr:Eu2+ storage phosphor particles, in binderless layers in the form of cylinders (and even up to a needle-shaped form) wherein said cylinder has an average cross-section diameter in the range from 1 micrometer (μm) to 30 μm (more preferred: from 2 μm up to 15 μm), an average length, measured along the casing of said cylinder, in the range from 100 μm up to 1000 μm (more preferred: from 100 μm up to 500 μm) as has e.g. been described in EP-A 1 359 204. Such block-shaped, prismatic, cylindrical or needle-shaped phosphors, whether or not obtained after milling, are, in another embodiment, coated in a phosphor binder layer.
Non-crystalline or amorphous europium activated alkaline earth metal halide phosphor screens advantageously have Ba(Sr)FBr:Eu2+ storage phosphor particles, dispersed in a binder medium in their corresponding storage phosphor layers.
The used phosphors are characterized by an afterglow emission with low amplitude but having a decay time that is large compared to the stimulation time of a single pixel, i.e. the period of time that a single pixel is stimulated by the scanning laser.
The photo-multiplier integrates all light that is directed towards its light sensitive surface. It thus detects the image-wise modulated light emitted by an instantaneously scanned pixel as well as the afterglow light emitted by previously scanned pixels.
Depending on the ratio of the decay time of the phosphor and the time the laser light is stimulating a pixel, the number of pixels that produce an afterglow that has an influence on the total amount of light that is detected for a certain pixel, can be determined.
Typically the scanning time of one pixel is about 1 μsec whereas the decay time of the phosphor ( . . . type phosphor) is 1 to 1.4 msec. This results in the fact that the afterglow produced by a scanned pixel still has an influence on the next 1000 scanned pixels.
This results in an asymmetric deformation of the detected signal which can have negative effects on the diagnosis to be performed on an image corresponding with the detected signal.
The negative effect can for example be seen on an image of a breast phantom such as the Thormamm phantom. The after glow of pixels outside the phantom image that were scanned prior to the pixels pertaining to the breast phantom have an influence on the pixels of the phantom, they increase the read out signal at the border of the phantom resulting in a lighter part in the image reproduction (hard copy image or displayed image). This phenomenon may have negative consequences on the diagnosis.
Also a lead-mark phantom positioned in the irradiated zone causes a line image to appear in the actual image.